cryptsetup/man/cryptsetup.8

.TH CRYPTSETUP "8" "May 2012" "cryptsetup" "Maintenance Commands"
.SH NAME
cryptsetup - manage plain dm-crypt and LUKS encrypted volumes
.SH SYNOPSIS
.B cryptsetup <options> <action> <action args>
.SH DESCRIPTION
.PP
cryptsetup is used to conveniently setup dm-crypt managed 
device-mapper mappings. These include plain dm-crypt volumes and 
LUKS volumes. The difference is that LUKS uses a metadata header
and can hence offer more features than plain dm-crypt. On the other 
hand, the header is visible and vulnerable to damage.
.SH PLAIN DM-CRYPT OR LUKS?
.PP
Unless you understand the cryptographic background well, use LUKS.
With plain dm-crypt there are a number of possible user errors
that massively decrease security. While LUKS cannot fix them
all, it can lessen the impact for many of them.
.SH WARNINGS
.PP 
A lot of good information on the risks of using encrypted storage,
on handling problems and on security aspects can be found in the
\fICryptsetup FAQ\fR. Read it. Nonetheless, some risks deserve
to be mentioned here. 

\fBBackup:\fR Storage media die. Encryption has no influence on that.
Backup is mandatory for encrypted data as well, if the data has any
worth. See the Cryptsetup FAQ for advice on how to do backup of an
encrypted volume.

\fBCharacter encoding:\fR If you enter a 
passphrase with special symbols, the passphrase can change 
depending character encoding. Keyboard settings can also change,
which can make blind input hard or impossible. For
example, switching from some ASCII 8-bit variant to UTF-8
can lead to a different binary encoding and hence different
passphrase seen by cryptsetup, even if what you see on
the terminal is exactly the same. It is therefore highly
recommended to select passphrase characters only from 7-bit 
ASCII, as the encoding for 7-bit ASCII stays the same for
all ASCII variants and UTF-8.

\fBLUKS header:\fR If the header of a LUKS volume gets damaged,
all data is permanently lost unless you have a header-backup.
If a key-slot is damaged, it can only be restored from a header-backup
or if another active key-slot with known passphrase is undamaged.
Damaging the LUKS header is something people manage to do with 
surprising frequency. This risk is the result of a trade-off 
between security and safety, as LUKS is designed for fast and 
secure wiping by just overwriting header and key-slot area. 

.SH PLAIN MODE
Plain dm-crypt encrypts the device sector-by-sector with a
single, non-salted hash of the passphrase. No checks 
are performed, no metadata is used. There is no formatting operation. 
When the raw device is mapped (created), the usual device operations 
can be used on the mapped device, including filesystem creation.
Mapped devices usually reside in /dev/mapper/<name>.

There are four operations:

\fIcreate\fR <name> <device>
.IP
Creates a mapping with <name> backed by device <device>.

\fB<options>\fR can be [\-\-hash, \-\-cipher, \-\-verify-passphrase,
\-\-key-file, \-\-keyfile-offset, \-\-key-size, \-\-offset, \-\-skip, \-\-size,
\-\-readonly, \-\-shared, \-\-allow-discards]

Example: 'cryptsetup create e1 /dev/sda10' maps the raw
encrypted device /dev/sda10 to the mapped (decrypted) device
/dev/mapper/e1, which can then be mounted, fsck-ed or have a 
filesystem created on it.
.PP
\fIremove\fR <name>
.IP
Removes the existing mapping <name> and wipes the key from kernel memory.
.PP
\fIstatus\fR <name>
.IP
Reports the status for the mapping <name>.
.PP
\fIresize\fR <name>
.IP
Resizes an active mapping <name>.

If \-\-size (in sectors) is not specified, the size of the 
underlying block device is used. Note that this does not
change the raw device geometry, it just changes how many
sectors of the raw device are represented in the mapped device.
.SH LUKS EXTENSION
LUKS, the Linux Unified Key Setup, is a standard for disk encryption.
It adds a standardized header at the start of the device, 
a key-slot area directly behind the header and the bulk
data area behind that. The whole set is called a 'LUKS container'.
The device that a LUKS container resides on is called a 'LUKS device'.
For most purposes both terms can be used interchangeably. But
note that when the LUKS header is at a nonzero offset
in a device, then the device is not a LUKS device anymore, but
has a LUKS container stored in it at an offset.

LUKS can manage multiple passphrases that can be individually revoked 
or changed and that can be securely scrubbed from persistent 
media due to the use of anti-forensic stripes. Passphrases
are protected against brute-force and dictionary
attacks by PBKDF2, which implements hash iteration and salting
in one function.

Each passphrase, also called a
.B key
in this document, is associated with one of 8 key-slots.
Key operations that do not specify a slot affect the first slot
that matches the supplied passphrase or the first empty slot if
a new passphrase is added.

The following are valid LUKS actions:

\fIluksFormat\fR <device> [<key file>]
.IP
Initializes a LUKS partition and sets the initial passphrase
(for key-slot 0), 
either via prompting or via <key file>. Note that 
if the second argument is present, then the passphrase 
is taken from the file given there, without the need
to use the \-\-key-file option. Also note that for both forms
of reading the passphrase from file you can 
give '-' as file name, which results in the passphrase being read
from stdin and the safety-question being skipped.

You can only call luksFormat on a LUKS device that is not mapped.

\fB<options>\fR can be [\-\-cipher, \-\-verify-passphrase, \-\-key-size,
\-\-key-slot, \-\-key-file (takes precedence over optional second argument),
\-\-keyfile-offset, \-\-keyfile-size, \-\-use-random | \-\-use-urandom, 
\-\-uuid,
\-\-master-key-file].

\fBWARNING:\fR Doing a luksFormat on an existing LUKS container will
make all data the old container permanently irretrievable, unless
you have a header backup.
.PP
\fIluksOpen\fR <device> <name>
.IP
Opens the LUKS device <device> and sets up a mapping <name> after
successful verification of the supplied passphrase. 
If the passphrase is not supplied via \-\-key-file, the command
prompts for it interactively.

The <device> parameter can be also specified by LUKS UUID in the 
format UUID=<uuid>, which uses the symlinks in /dev/disk/by-uuid.

\fB<options>\fR can be [\-\-key-file, \-\-keyfile-offset, 
\-\-keyfile-size, \-\-readonly, \-\-test-passphrase,
\-\-allow-discards, \-\-header, \-\-key-slot, \-\-master-key-file].
.PP
\fIluksClose\fR <name>
.IP
identical to \fIremove\fR.
.PP
\fIluksSuspend\fR <name>
.IP
Suspends an active device (all IO operations will blocked
and accesses to the device will wait indefinitely)
and wipes the encryption
key from kernel memory. Needs kernel 2.6.19 or later.

After this operation you have to use \fIluksResume\fR to reinstate
the encryption key and unblock the device or \fIluksClose\fR to remove 
the mapped device.

\fBWARNING:\fR never suspend the device on which the cryptsetup binary resides.

\fB<options>\fR can be [\-\-header].
.PP
\fIluksResume\fR <name>
.IP
Resumes a suspended device and reinstates the encryption key. 
Prompts interactively for a passphrase if \-\-key-file is not given.

\fB<options>\fR can be [\-\-key-file, \-\-keyfile-size, \-\-header]
.PP
\fIluksAddKey\fR <device> [<key file with new key>]
.IP
adds a new passphrase. An existing passphrase must be supplied 
interactively or via \-\-key-file.
The new passphrase to be added can be specified interactively
or read from the file given as positional argument.

\fB<options>\fR can be [\-\-key-file, \-\-keyfile-offset, 
\-\-keyfile-size, \-\-new-keyfile-offset,
\-\-new-keyfile-size, \-\-key-slot, \-\-master-key-file].
.PP
\fIluksRemoveKey\fR <device> [<key file with passphrase to be removed>] 
.IP
Removes the supplied passphrase from the LUKS device. The
passphrase to be removed can be specified interactively,
as positional argument or via \-\-key-file.

\fB<options>\fR can be [\-\-key-file, \-\-keyfile-offset, 
\-\-keyfile-size]

\fBWARNING:\fR If you read the passphrase from stdin
(without further argument or with '-' as argument 
to \-\-key-file), batch-mode (-q) will be implicitely
switched on and no warning will be given when you remove the 
last remaining passphrase from a LUKS container. Removing
the last passphrase makes the LUKS container permanently 
inaccessible.
.PP
\fIluksChangeKey\fR <device> [<new key file>]
.IP
Changes an existing passphrase. The passphrase
to be changed must be supplied interactively or via \-\-key-file.
The new passphrase can be supplied interactively or in 
a file given as positional argument.

If a key-slot is specified (via \-\-key-slot), the passphrase
for that key-slot must be given and the new passphrase
will overwrite the specified key-slot. If no key-slot
is specified and there is still a free key-slot, then
the new passphrase will be put into a free key-slot before the 
key-slot containing the old passphrase is purged. If there is
no free key-slot, then the key-slot with the old passphrase is
overwritten directly.

\fBWARNING:\fR If a key-slot is overwritten, a media failure
during this operation can cause the overwrite to fail after
the old passphrase has been wiped and make the LUKS container
inaccessible. 

\fB<options>\fR can be [\-\-key-file, \-\-keyfile-offset, 
\-\-keyfile-size, \-\-new-keyfile-offset,
\-\-new-keyfile-size, \-\-key-slot].
.PP
\fIluksKillSlot\fR <device> <key slot number>
.IP
Wipe the key-slot number <key slot> from the LUKS device. A remaining 
passphrase must be supplied, either interactively or via \-\-key-file.
This command can remove the last remaining key-slot, but requires
an interactive confirmation when doing so. Removing the last
passphrase makes a LUKS container permanently inaccessible.

\fB<options>\fR can be [\-\-key-file, \-\-keyfile-offset, \-\-keyfile-size].

\fBWARNING:\fR If you read the passphrase from stdin
(without further argument or with '-' as argument 
to \-\-key-file), batch-mode (-q) will be implicitely
switched on and no warning will be given when you remove the 
last remaining passphrase from a LUKS container. Removing
the last passphrase makes the LUKS container permanently 
inaccessible.
.PP
\fIluksUUID\fR <device>
.IP
Print the UUID of a LUKS device.
.br
Set new UUID if \fI\-\-uuid\fR option is specified.
.PP
\fIisLuks\fR <device>
.IP
Returns true, if <device> is a LUKS device, false otherwise.
Use option \-v to get human-readable feedback. 'Command successful.'
means the device is a LUKS device. 
.PP
\fIluksDump\fR <device>
.IP
Dump the header information of a LUKS device.

If the \-\-dump-master-key option is used, the LUKS device master key is 
dumped instead of the keyslot info. Beware that the master key cannot be
changed and can be used to decrypt the data stored in the LUKS container
without a passphrase and even without the LUKS header. This means
that if the master key is compromised, the whole device has to be 
erased to prevent further access. Use this option carefully.  

In order to dump the master key, a passphrase has to be supplied,
either interactively or via \-\-key-file. 

\fB<options>\fR can be [\-\-dump-master-key, \-\-key-file, 
\-\-keyfile-offset, \-\-keyfile-size].

\fBWARNING:\fR If \-\-dump-master-key is used with \-\-key-file
and the argument to \-\-key-file is '-', no validation question 
will be asked and no warning given.
.PP
\fIluksHeaderBackup\fR <device> \-\-header-backup-file <file>
.IP
Stores a binary backup of the LUKS header and keyslot area.
.br
Note: Using '-' as filename writes the header backup to a file named '-'. 

\fBWARNING:\fR This backup file and a passphrase valid
at the time of backup allows decryption of the 
LUKS data area, even if the passphrase was later changed or 
removed from the LUKS device. Also note that with a header
backup you lose the ability to securely wipe the LUKS
device by just overwriting the header and key-slots. You 
either need to securely erase all header backups in 
addition or overwrite the encrypted data area as well.
The second option is less secure, as some sectors
can survive, e.g. due to defect management. 
.PP
\fIluksHeaderRestore\fR <device> \-\-header-backup-file <file>
.IP
Restores a binary backup of the LUKS header and keyslot area 
from the specified file.
.br
Note: Using '-' as filename reads the header backup from a file named '-'.

\fBWARNING:\fR Header and keyslots will be replaced, only
the passphrases from the backup will work afterwards.

This command requires that the master key size and data offset 
of the LUKS header already on the device and of the header backup 
match. Alternatively, if there is no LUKS header on the device,
the backup will also be written to it.
.PP
\fIrepair\fR <device>
.IP
Tries to repair the LUKS device metadata if possible.

This command is useful to fix some known benign LUKS metadata 
header corruptions. Only basic corruptions of unused keyslot 
are fixable. This command will only change the LUKS header, not
any key-slot data.

\fBWARNING:\fR Always create a binary backup of the original 
header before calling this command.
.SH loop-AES EXTENSION
cryptsetup supports mapping loop-AES encrypted partition using
a compatibility mode.
.PP
\fIloopaesOpen\fR <device> <name> \-\-key-file <keyfile>
.IP
Opens the loop-AES <device> and sets up a mapping <name>.

If the key file is encrypted with GnuPG, then you have to use
\-\-key-file=- and decrypt it before use, e.g. like this:
.br
gpg \-\-decrypt <keyfile> | cryptsetup loopaesOpen \-\-key-file=- <device> <name>

Use \fB\-\-key-file-size\fR to specify the proper key length if needed. 

Use \fB\-\-offset\fR to specify device offset. Note that the units 
need to be specified in number of 512 byte sectors.

Use \fB\-\-skip\fR to specify the IV offset. If the original device 
used an offset and but did not use it in IV sector calculations, 
you have to explicitly use \fB\-\-skip 0\fR in addition to the offset 
parameter.

Use \fB\-\-hash\fR to override the default hash function for 
passphrase hashing (otherwise it is detected according to key 
size).

\fB<options>\fR can be [\-\-key-file, \-\-key-size, \-\-offset, \-\-skip,
\-\-hash, \-\-readonly, \-\-allow-discards].
.PP
\fIloopaesClose\fR <name>
.IP
Identical to \fIremove\fR.
.PP
See also section 7 of the FAQ and \fBhttp://loop-aes.sourceforge.net\fR
for more information regarding loop-AES.
.SH MISCELLANEOUS
.PP
\fIbenchmark\fR <options>
.IP
Benchmarks ciphers. Without parameters it tries to measure few common
configurations.

To benchmark other ciphers or modes, you need to specify \fB\-\-cipher\fR
and \fB\-\-key-size\fR options.

\fBNOTE:\fR This benchmark is using memory only and is only informative.
You cannot directly predict real storage encryption speed from it.

This benchmark requires kernel userspace crypto API interface to be available
(kernel af_alg and af_skcipher modules, introduced in Linux kernel 2.6.38).

\fB<options>\fR can be [\-\-cipher, \-\-key-size].
.SH OPTIONS
.TP
.B "\-\-verbose, \-v"
Print more information on command execution.
.TP
.B "\-\-debug"
Run in debug mode with full diagnostic logs. Debug output
lines are always prefixed by '#'.
.TP
.B "\-\-hash, \-h \fI<hash-spec>\fR"
Specifies the passphrase hash for \fIcreate\fR and \fIloopaesOpen\fR.

Specifies the hash used in the LUKS key setup scheme and volume key digest 
for \fIluksFormat\fR.

The specified hash name is passed to the compiled-in crypto backend. 
Different backends may support different hashes. 
For \fIluksFormat\fR, the hash 
algorithm must provide at least 160 bits of output, which 
excludes, e.g., MD5. Do not use a non-crypto hash like
\fB"crc32"\fR as this breaks security.

Values compatible with old version of cryptsetup are 
\fB"ripemd160"\fR for \fIcreate\fR and 
\fB"sha1"\fR for \fIluksFormat\fR.

Use \fIcryptsetup \-\-help\fR to show the defaults.
.TP
.B "\-\-cipher, \-c \fI<cipher-spec>\fR"
Set the cipher specification string.

\fIcryptsetup \-\-help\fR shows the compiled-in defaults.
The current default in the distributed sources is
"aes-cbc-essiv:sha256" for both plain dm-crypt and LUKS.

For XTS mode (a possible future default), use "aes-xts-plain"
or better "aes-xts-plain64"
as cipher specification and optionally set a key size of
512 bits with the \-s option. Key size for XTS
mode is twice that for other modes for the same 
security level. 

XTS mode requires kernel 2.6.24 or later and plain64 requires 
kernel 2.6.33 or later. More information can be found in the FAQ.
.TP
.B "\-\-verify-passphrase, \-y"
When interactively asking for a passphrase, ask for it twice
and complain if both inputs do not match. Advised when creating 
a regular mapping for the first time, or when running 
\fIluksFormat\fR. Ignores on input from file or stdin.
.TP
.B "\-\-key-file, \-d \fIname\fR"
Read the passphrase from file.

If the name given is "-", then the passphrase will be read from stdin. 
In this case, reading will not stop at newline characters.

With LUKS, passphrases supplied via \-\-key-file are always 
the existing passphrases requested by a command, except in 
the case of \fIluksFormat\fR where \-\-key-file is equivalent 
to the positional key file argument.

If you want to set a new passphrase via key file, you have to 
use a positional argument to \fIluksAddKey\fR.

See section \fBNOTES ON PASSPHRASE PROCESSING\fR for more information.
.TP
.B "\-\-keyfile-offset \fIvalue\fR"
Skip \fIvalue\fR bytes at the beginning of the key file.
Works with all commands that accepts key files.
.TP
.B "\-\-keyfile-size, \-l \fIvalue\fR"
Read a maximum of \fIvalue\fR bytes from the key file.
Default is to read the whole file up to the compiled-in
maximum that can be queried with \-\-help. Supplying more 
data than the compiled-in maximum aborts the operation.

This option is useful
to cut trailing newlines, for example. If \-\-keyfile-offset 
is also given, the size count starts after the offset.
Works with all commands that accepts key files.
.TP
.B "\-\-new-keyfile-offset \fIvalue\fR"
Skip \fIvalue\fR bytes at the start when
adding a new passphrase from key file with 
\fIluksAddKey\fR. 
.TP
.B "\-\-new-keyfile-size  \fIvalue\fR"
Read a maximum of \fIvalue\fR bytes when adding
a new passphrase from key file with \fIluksAddKey\fR. 
Default is to read the whole file up to the compiled-in 
maximum length that can be queried with \-\-help.
Supplying more than the compiled in maximum aborts the
operation.
When \-\-new-keyfile-offset is also given, reading starts
after the offset.
.TP
.B "\-\-master-key-file"
Use a master key stored in a file. 

For \fIluksFormat\fR this
allows creating a LUKS header with this specific
master key. If the master key was taken from an existing
LUKS header and all other parameters are the same,
then the new header decrypts the data encrypted with the
header the master key was taken from.

For \fIluksAddKey\fR this allows adding a new passphrase 
without having to know an exiting one.

For \fIluksOpen\fR this allows to open the LUKS device 
without giving a passphrase.
.TP
.B "\-\-dump-master-key"
For \fIluksDump\fR this option includes the master key in the displayed
information. Use with care, as the master key can be used to
bypass the passphrases, see also option \-\-master-key-file. 
.TP
.B "\-\-use-random"
.TP
.B "\-\-use-urandom"
For \fIluksFormat\fR these options define which kernel random number 
generator will be used to create the master key (which is a
long-term key).

See \fBNOTES ON RANDOM NUMBER GENERATORS\fR for more 
information. Use \fIcryptsetup \-\-help\fR
to show the compiled-in default random number generator.

\fBWARNING:\fR In a low-entropy situation (e.g. in an 
embedded system), both selections are problematic.
Using /dev/urandom can lead to weak keys.
Using /dev/random can block a long time, potentially 
forever, if not enough entropy can be harvested by 
the kernel.
.TP
.B "\-\-key-slot, \-S <0-7>"
For LUKS operations that add key material, this options allows you
to specify which key slot is selected for the new key.
This option can be used for \fIluksFormat\fR, 
and \fIluksAddKey\fR.
.br
In addition, for \fIluksOpen\fR, this option selects a
specific key-slot to compare the passphrase against.
If the given passphrase would only match a different key-slot,
the operation fails.            
.TP
.B "\-\-key-size, \-s <bits>"
Sets key size in bits. The argument has to be a multiple of 
8. The possible key-sizes are limited by the cipher and
mode used. 

See /proc/crypto for more information. Note that key-size 
in /proc/crypto is stated in bytes.

This option can be used for \fIcreate\fR or \fIluksFormat\fR. All 
other LUKS actions will use the key-size specified in the LUKS header.
Use \fIcryptsetup \-\-help\fR to show the compiled-in defaults.
.TP
.B "\-\-size, \-b <number of 512 byte sectors>"
Force the size of the underlying device in sectors of 512 bytes.
This option is only relevant for the \fIcreate\fR and \fIresize\fR 
actions. 
.TP
.B "\-\-offset, \-o <number of 512 byte sectors>"
Start offset in the backend device in 512-byte sectors.
This option is only relevant for the \fIcreate\fR and \fIloopaesOpen\fR 
actions.
.TP
.B "\-\-skip, \-p <number of 512 byte sectors>"
How many sectors of the encrypted data to skip at the beginning.
This option is only relevant for \fIcreate\fR and \fIloopaesOpen\fR action.

This is different from the \-\-offset options with respect to 
the sector numbers used in IV calculation.
Using \-\-offset will shift the IV calculation by the same negative amount.
Hence, if \-\-offset \fIn\fR, sector \fIn\fR will get a sector
number of \fI0\fR for the IV calculation.
Using \-\-skip causes sector \fIn\fR to also be the first sector
of the mapped device, but with its number for IV generation is \fIn\fR.
.TP
.B "\-\-readonly, \-r" 
set up a read-only mapping.
.TP
.B "\-\-shared"
Creates an additional mapping for one common 
ciphertext device. Arbitrary mappings are supported.
This option is only relevant for the 
\fIcreate\fR action. Use \-\-offset, \-\-size and \-\-skip to 
specify the mapped area.
.TP
.B "\-\-iter-time, \-i <number of milliseconds>"
The number of milliseconds to spend with PBKDF2 passphrase processing.
This option is only relevant for LUKS operations that set or change
passphrases, such as \fIluksFormat\fR or \fIluksAddKey\fR.
Specifying 0 as parameter selects the compiled-in default.
.TP
.B "\-\-batch-mode, \-q"
Suppresses all confirmation questions. Use with care! 

If the \-y option is not specified, this option also switches off
the passphrase verification for \fIluksFormat\fR.
.TP
.B "\-\-timeout, \-t <number of seconds>"
The number of seconds to wait before timeout on passphrase input
via terminal. It is relevant every time a passphrase is asked, 
for example for \fIcreate\fR, \fIluksOpen\fR, \fIluksFormat\fR 
or \fIluksAddKey\fR. It has no effect if used in conjunction 
with \-\-key-file. 
.br
This option is useful when the system 
should not stall if the user does not input a passphrase, 
e.g. during boot. The default is a value of 0 seconds,
which means to wait forever.  
.TP
.B "\-\-tries, \-T"
How often the input of the passphrase shall be retried. 
This option is relevant
every time a passphrase is asked, for example for 
\fIcreate\fR, \fIluksOpen\fR, \fIluksFormat\fR
or \fIluksAddKey\fR. The default is 3 tries.
.TP
.B "\-\-align-payload <number of 512 byte sectors>"   
Align payload at a boundary of \fIvalue\fR 512-byte sectors.
This option is relevant for \fIluksFormat\fR.

If not specified, cryptsetup tries to use the topology info 
provided by kernel for the underlying device to get optimal alignment.
If not available (or the calculated value is a multiple of the default) 
data is by default aligned to a 1MiB boundary (i.e. 2048 512-byte sectors).

For a detached LUKS header this option specifies the offset on the
data device. See also the \-\-header option.
.TP
.B "\-\-uuid=\fIUUID\fR"
Use the provided \fIUUID\fR for the \fIluksFormat\fR command 
instead of generating new one. Changes the existing UUID when 
used with the \fIluksUUID\fR command.

The UUID must be provided in the standard UUID format,
e.g. 12345678-1234-1234-1234-123456789abc.
.TP
.B "\-\-allow-discards\fR"
Allow the use of discard (TRIM) requests for device.
This option is only relevant for \fIcreate\fR, \fIluksOpen\fR 
and \fIloopaesOpen\fR.

\fBWARNING:\fR This command can have a negative security impact
because it can make filesystem-level operations visible on
the physical device. For example, information leaking
filesystem type, used space, etc. may be extractable from
the physical device if the discarded blocks can be located
later. If in doubt, do no use it.

A kernel version of 3.1 or later is needed. For earlier kernels
this option is ignored.
.TP
.B "\-\-test-passphrase\fR"
Do not activate device, just verify passphrase.
This option is only relevant for \fIluksOpen\fR.
.TP
.B "\-\-header\fR <device or file storing the LUKS header>"
Use a detached (separated) metadata device or file where the 
LUKS header is stored. This options allows to store ciphertext
and LUKS header on different devices.

This option is only relevant for LUKS devices and can be 
used with the \fIluksFormat\fR, \fIluksOpen\fR, \fIluksSuspend\fR, 
\fIluksResume\fR, \fIstatus\fR and \fIresize\fR commands.

For \fIluksFormat\fR with a file name as argument to \-\-header,
it has to exist and be large enough to contain the LUKS header.
See the cryptsetup FAQ for header size calculation.

For other commands that change the LUKS header (e.g. \fIluksAddKey\fR),
specify the device or file with the LUKS header directly as the 
LUKS device.

If used with \fIluksFormat\fR, the \-\-align-payload option is taken
as absolute sector alignment on ciphertext device and can be zero.

\fBWARNING:\fR There is no check whether the ciphertext device specified
actually belongs to the header given. In fact you can specify an
arbitrary device as the ciphertext device for \fIluksOpen\fR
with the \-\-header option. Use with care.
.TP
.B "\-\-version"
Show the program version.
.SH RETURN CODES
Cryptsetup returns 0 on success and a non-zero value on error.

Error codes are: 1 wrong parameters, 2 no permission (bad passphrase),
3 out of memory, 4 wrong device specified, 5 device already exists
or device is busy.
.SH NOTES ON PASSPHRASE PROCESSING FOR PLAIN MODE
Note that no iterated hashing or salting is done in plain mode.
If hashing is done, it is a single direct hash. This means that
low-entropy passphrases are easy to attack in plain mode.

\fBFrom a terminal\fR: The passphrase is read until the
first newline, i.e. '\\n'.
The input without the newline character is processed with
the default hash or the hash specified with \-\-hash.
The has result will be truncated to the key size 
of the used cipher, or the size specified with \-s.

\fBFrom stdin\fR: Reading will continue until a newline (or until
the maximum input size is reached), with the trailing newline 
stripped. The maximum input size is defined by the same 
compiled-in default as for the maximum key file size and  can 
be overwritten using \-\-keyfile-size option.

The data read will be hashed with the default hash
or the hash specified with \-\-hash.
The has result will be truncated to the key size
of the used cipher, or the size specified with \-s.

Note that if \-\-key-file=- is used for reading the key 
from stdin, trailing newlines are not stripped from the input. 

If "plain" is used as argument to \-\-hash, the input
data will not be hashed. Instead, it will be zero padded (if 
shorter than the key size) or truncated (if longer than the 
key size) and used directly as the binary key. This is useful for
directly specifying a binary key.
No warning will be given if the amount of data read from stdin is
less than the key size.

\fBFrom a key file\fR: It will be truncated to the 
key size of the used cipher or the size given by \-s
and directly used as binary key. 
if the key file is shorter than the key, cryptsetup
will quit with an error.

.SH NOTES ON PASSPHRASE PROCESSING FOR LUKS
LUKS uses PBKDF2 to protect against dictionary attacks 
and to give some protection to low-entropy passphrases
(see RFC 2898 and the cryptsetup FAQ).

\fBFrom a terminal\fR: The passphrase is read until the
first newline and then processed by PBKDF2 without 
the newline character.

\fBFrom stdin\fR:
LUKS will read passphrases from stdin up to the 
first newline character or the compiled-in
maximum key file length. If \-\-keyfile-size is 
given, it is ignored. 

\fBFrom key file\fR:
The complete keyfile is read up to the compiled-in
maximum size. Newline characters do not terminate the
input. The \-\-keyfile-size option can be used to limit
what is read.

\fBPassphrase processing\fR:
Whenever a passphrase is added to a LUKS header (luksAddKey, luksFormat),
the user may specify how much the time the passphrase processing
should consume. The time is used to determine the iteration count 
for PBKDF2 and higher times will offer better protection for
low-entropy passphrases, but luksOpen will take longer to 
complete. For passphrases that have entropy higher than the
used key length, higher iteration times will not increase security.

The default setting of one second is sufficient for most
practical cases. The only exception is a low-entropy
passphrase used on a slow device.
.SH INCOHERENT BEHAVIOR FOR INVALID PASSPHRASES/KEYS
LUKS checks for a valid passphrase when an encrypted partition
is unlocked. The behavior of plain dm-crypt is different.
It will always decrypt with the passphrase given. If the
given passphrase is wrong, the device mapped by plain
dm-crypt will essentially still contain encrypted data and 
will be unreadable.
.SH NOTES ON SUPPORTED CIPHERS, MODES, HASHES AND KEY SIZES
The available combinations of ciphers, modes, hashes and key sizes
depend on kernel support. See /proc/crypto for a list of available
options. You might need to load additional kernel crypto modules
in order to get more options.

For the \-\-hash option, if the crypto backend is libgcrypt, 
then all algorithms supported by the gcrypt library are available.
For other crypto backends some algorithms may be missing.
.SH NOTES ON PASSPHRASES
Mathematics can't be bribed. Make sure you keep your passphrases safe.
There are a few nice tricks for constructing a fallback, when suddenly
out of the blue, your brain refuses to cooperate.
These fallbacks need LUKS, as it's only possible with LUKS
to have multiple passphrases. Still, if your attacker model does
not prevent it, storing your passphrase in a sealed envelope somewhere 
may be a good idea as well.
.SH NOTES ON RANDOM NUMBER GENERATORS
Random Number Generators (RNG) used in cryptsetup are always the 
kernel RNGs without any modifications or additions to data stream 
produced.

There are two types of randomness cryptsetup/LUKS needs. One type 
(which always uses /dev/urandom) is used for salts, the AF splitter 
and for wiping deleted keyslots.

The second type is used for the volume (master) key. You can switch 
between using /dev/random and /dev/urandom  here, see 
\fP\-\-use-random\fR and \fP\-\-use-urandom\fR
options. Using /dev/random on a system without enough entropy sources
can cause \fPluksFormat\fR to block until the requested amount of 
random data is gathered. In a low-entropy situation (embedded system),
this can take a very long time and potentially forever. At the same
time, using /dev/urandom in a low-entropy situation will 
produce low-quality keys. This is a serious problem, but solving
it is out of scope for a mere man-page.
See \fPurandom(4)\fR for more information.
.SH NOTES ON LOOPBACK DEVICE USE
Cryptsetup is usually used directly on a block device (disk 
partition or LVM volume). However, if the device argument is a 
file, cryptsetup tries to allocate a loopback device
and map it into this file. This mode requires Linux kernel 2.6.25 
or more recent which supports the loop autoclear flag (loop device is 
cleared on last close automatically). Of course, you can
always map a file to a loop-device manually. See the 
cryptsetup FAQ for an example.

When device mapping is active, you can see the loop backing file in 
the status command output. Also see losetup(8).
.SH DEPRECATED ACTIONS
.PP
The \fIreload\fR action is no longer supported.
Please use \fIdmsetup(8)\fR if you need to
directly manipulate with the device mapping table.
.PP
The \fIluksDelKey\fR was replaced with \fIluksKillSlot\fR.
.PP
.SH REPORTING BUGS
Report bugs, including ones in the documentation, on
the cryptsetup mailing list at <dm-crypt@saout.de> 
or in the 'Issues' section on LUKS website.
Please attach the output of the failed command with the 
\-\-debug option added.
.SH AUTHORS
cryptsetup originally written by Christophe Saout <christophe@saout.de>
.br
The LUKS extensions and original man page were written by 
Clemens Fruhwirth <clemens@endorphin.org>.
.br
Man page extensions by Milan Broz <gmazyland@gmail.com>.
.br
Man page rewrite and extension by Arno Wagner <arno@wagner.name>. 
.SH COPYRIGHT
Copyright \(co 2004 Christophe Saout
.br
Copyright \(co 2004-2006 Clemens Fruhwirth
.br
Copyright \(co 2009-2012 Red Hat, Inc.
.br
Copyright \(co 2012 Arno Wagner

This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
.SH SEE ALSO
The LUKS website at \fBhttp://code.google.com/p/cryptsetup/\fR

The cryptsetup FAQ, contained in the distribution package and
online at 
\fBhttp://code.google.com/p/cryptsetup/wiki/FrequentlyAskedQuestions\fR

The cryptsetup mailing list and list archive, see FAQ entry 1.6.

The LUKS on-disk format specification available at
\fBhttp://code.google.com/p/cryptsetup/wiki/Specification\fR